Electric contact converters



April 8,1958 5, ROLF Re. 24,456

ELECTRIC CONTACT CONVERTERS Original Filed March 25, 1952 8 Sheets-Sheet1 LOAD " FIG.2-

April 8, 1958- E. ROLF Re. 24,456

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April 8, 1958 E. ROLF 24,456

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8 Sheets-Sheet 5 April 8, 1958 Y E. ROLF Re. 24,456

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I ELECTRIC CONTACT CONVERTERS Original Filed March 25,. 1952 8Sheets-Sheet 8 United States Patent ELECTRIC CONTACT CONVERTERS ErichRolf, Nurnberg, Germany, assignor to Siemens- SchuckertwerkeAktiengesellschaft, Berlin-Siemensstadt, Germany, a German corporationOriginal No. 2,756,381, dated July 24, 1956, Serial No. 278,385, March25, 1952. Application for reissue June 20, 1957, Serial No. 668,011

Claims priority, appiication Germany March 36, 1951 50 Claims. (Cl.321-48) My invention relates to electric contact converters whosesynchronous contact devices are series' connected with saturablecommutating reactors serving to temporarily minimize the instantaneouscurrent during a recurrent interval of time within which the contactdevices may open without sparking.

In such converters, the contact devices are either actuated by asynchronous motor energized from the alternating-current supply, or theyare controlled by electromagnetic means. In the former case, therecurrent clos ing moments of the converter contacts are generallyadjustable relative to the alternating-current cycle to permitregulating the converter output voltage in accordance with thedelayed-commutation method. In contrast thereto, the opening'moment ofthe electro-magnetically controlled contacts occurs always at the zeropassage of the contact current, thus automatically adapting itself tothe duration of the natural current-conducting interval of the contactwithin each cycle period.

In one type of magnetically controlled contact converters, the movablecontact armature is held in the opening or closing position by means ofpermanent magnets; and the closing or opening movement of the contact iscontrolled by current pulses which temporarily counteract the holdingforce of the permanent magnet to let the movable contact armature swingto its other position (pulse-controlled switches). In another type ofmagnetically controlled contact converters the contact current itselftakes care of electromagnetically holding the armature in the closingposition. During closing, and in some cases also during openingperformance, the contact current is replaced or aided in this operationby an auxiliary current flowing through an electric valve circuit acrossthe contact gap, or/and by current pulses that occur at the propermoment (load-current controlled switches).

In all mentioned contact converters, the series-connected commutationreactors have a high reactance only temporarily during the commutationintervals when the magnetic flux in the reactor core changes betweenopposingly directed saturation values, While the reactance is negligiblysmall at all other times when the core is saturated. As a result, thereactor depresses the current to small instantaneous current values inthe neighborhood of a current zero passage thus flattening the currentwave to a. step of a very small residual current magnitude (stepcurrent). Referring to the customary power line frequencies of 50 or 60C. P. S., the step interval may last some tenths of one millisecond inmagnetically controlled converters, and about 1 to 2 milliseconds inmotoractuated converters. To have the opening of the contact occurpractically without any transfer of contact material, the step currentflowing during the step interval through the contact at the openingmoment should be as close as possible to zero or should, in any event,not exceed a few tenths of one ampere. Since the natural magnitude ofthe step current, which represents the mag netizing current of thecommutating reactor, lies considerably above this limit especially withconverters of large power ratings, it is an important problem in thedesign of contact converters to satisfy the requirement for apractically current-free contact opening operation for all occurringvariations in load conditions. For this purpose, a suitablepremagnetization may be applied to the commutation reactor. However thepremagnetizing circuit means heretofore known leave much to de desiredand solve this problem only within certain limits.

It is therefore an object of my invention to provide a contact converterwith premagnetizing circuit means' for the pertaining commutatingreactor which afford an automatic adaptation of the premagnetizing fluxin the reactor under all possible operating conditions so that thecontact current is always practically zero at the opening moment.

The invention is based upon the concept of making the premagnetizationof the commutating reactor dependent upon the voltage which obtainsbetween theterminal of the pertaining transformer winding and theconverter load and hence is effective across the series connection ofcommutating reactor and converter contact. This particular voltage isidentical with the voltage e that during the current step is impressedacross the commutating reactor and thus determines the premagnetizingvelocity dB/dt of the reactor in accordance with the equation:

In this equation, w is the number of turns of the reactance winding(main winding) of the reactor, q the iron cross section of the reactorcore, and B is the magnetic inductance, while t denotes time. Thecoercive force H,, of the reactor iron, being a measure for themagnitude of the natural step current i of the reactor, is a function ofthe magnetizing velocity dB/a't and hence of the proportional reactorvoltage e as is apparent from the coordinate diagram shown in Fig. 1.Starting from'the static value H for dB/dt=0, the curve of the coerciveforce H rises at first rather steeply and then enters into a muchflatter and nearly linear portion at a still relatively small value ofthe premagnetizing velocity dB/dt. Such a course applies not only to thecoercive force, i. e. to the flank midportion of the hysteresis loop(B=0), but also to other field-strength values along the flank of thehysteresis loop, particularly when the shape of the flank is flattenedby a shaping circuit so that the flank extends approximately parallel tothe ordinate axis. Such flattening or shaping circuits, known as such,consist of the series connection of a capacitor with an ohmic resistoror/and a damped oscillatory circuit, and are connected either directlyacross the reactor main Winding or across an auxiliary win-ding on thereactor core (see 17 in Figs. 2, 4, 5, l4, 16 to 18). Such animprovement of the loop shape by means of shaping circuits also resultsin producing a practically horizontal course of the current step. Ifunder these conditions the step current, flowing through the contactduring the step interval, is to be reduced to zero, then the naturalstep current i for any occurring value of the voltage e must becompensated by a premagnetizing current i, that satisfies the condition:

w LI-1.5;;

wherein w denotes the number of turns of the premagne- Reissued Apr. 8,1958 must be made dependent upon the premagnetizing velocitycorresponding to the curve of Fig. 1 or to a similar mathematicfunction. In most cases the bent portion of the curve, applying to smallvalues of premagnetizing velocity, need not be considered because suchsmall velocity values do not normally occur within the interval of timeduring which the contact opening may take place. The requireddependence, therefore, is practically attained if the curve of Fig. 1 issubstituted by a straight line approximately corresponding to the uppercurve branch whose extrapolation, shown in Fig. 1 by a broken line,intersects the ordinate at the base of value H According to a morespecific feature of my invention, therefore, the total premagnetizationof the commutating reactors is composed of two superimposed componentsof which one is constant during the step interval while the other variesin dependence upon the voltage obtaining during the step interval acrossthe commutating reactor. It is particularly advantageous to have theconstant component correspond to the base value H (Fig. 1) and to makethe voltage-responsive component proportional to the reactor voltage 2so that it corresponds to the value H in Fig. 1. This secures the mostfavorable operating condition. The constant component is supplied, forinstance, by' a premagnetization with direct current which may beinductively stabilized against transformer reaction of the reactor mainwinding. The volt age-responsive component, however, is supplied by apremagnetizing winding which is connected between the winding terminalof the pertaining power-supply transformer and the direct-current sideof the converter contact and which therefore carries a currentproportional to the voltage eifective between these two connectionpoints. The coaction of both premagnctizing components (magnetomotiveforces) results in the desired dependence of the total reactorpremagnetization upon the premagnetizing velocity, corresponding to theupper curve portion in Fig. l.

This effect is not predicated upon any condition as regards magnitudeand time dependency of the voltage across the reactor. Consequently, thedevice adapts itself automatically to all occurring conditions ofconverter operation. This automatic adaptation also takes care ofcompensating for voltage fluctuations of the alternating-current supplyline as well as for any intentional or desired voltage changes as may becaused by transformer-tap switching or by the operation of any othervoltage regulating or control means. Besides, the power losses in thepremagnetizing circuits are smaller than with the known premagnetizingcircuits in which the premagnetizing winding of the reactor are excitedthrough a resistor by a voltage composed of the cross-phase transformervoltage and an additional series voltage. It will be recognized that byvirtue of the reactor premagnetiration according to the invention, theconverter contacts open practically without current and without voltageat the end of their current-conducting periods regardless of any changesin converter operation.

According to another object, my invention also aims at improving thecurrent-step conditions (make step) preveiling at the time of thecontact closing operations. To this end, and in accordance with anotherfeature of my invention, the premagnetization of the cornmutatingreactor at the time of the make operation (make premagnetization) isalso composed of two component electromotive forces of which one remainsconstant during the make step interval While the other varies independence upon the voltage across the series connection of convertercontact and 'commutating reactor. This aftords having the contacts closepractically without current and without voltage -at the beginning oftheir respective current-conducting periods.

The constant component of the make premagnetization maybe produced bypassing a direct current through an auxiliary reactor winding asdescribed above with reference to the break premagnetization. However,while the direct-current flux of the auxiliary winding for the breakpremagnetization has a magnetizing direction opposed to that of thereactor main winding, the magnetizing direction of the auxiliary windingfor the make premagnetiza- I tion must have the same magnetizing senseas the main winding and may require a current magnitude different fromthat applied to the break premagnetizatiou winding.

Further problems arise in special cases, for instance, when therectified voltage delivered by the converter is to be controlled orregulated by the mechanical commutation-delay method, i. e. by delayingthe contact closing moment a desired control angle relative to the zeropassage of the alternating voltage. When the contact closing moment isthus delayed, the commutating reactor, under the efiect of thepositively directed alternating voltage, may be premagnetized up tosaturation already prior to the closing moment. The make step would thenbe terminated ahead of the contact closing moment and would no longer beavailable for its intended purpose of initially preventing the contactcurrent from rapidly rising immediately upon the initial closing touchof the contact. Besides, when a single commutating reactor serves toproduce the make step as well as the break step, a breakpremagnetization with direct current would have the wrong direction forthe make operation. it is, therefore, also among the objects of myinvention to obviate these various difficulties. To this end, and inaccordance with further features of my invention, the following meansare applicable.

To make certain that, when providing a single commutating reactor, theconstant component of premagnetization 'has the required directionduring the make performance as well as during the break performance,this component is preferably applied as a premagnctizing magnetomotiveforce of alternating direction. This magnetomotive force, particularlyfor reacting requirements, may have a rectangular or trapezoidal Waveshape. In

certain cases, especially for moderate requirements suchas a small ratedpower and a small regulating range, it is also possible to supply theconstant component as a sinusoidal current, it being understood thatthis current has a single given polarity during each of the respectivemake and break step intervals. The rectangular or trapezoidal waveshapes may be produced in the known 1nanme by distorting an originallysinusoidal current with the aid of series connected saturable reactors(transductors) magnetically excited by direct current. Another suitableway of providing currents of trapezoidal wave shape to supply them asthe anode or transformer currents in a rectifier circuit connection. itis also possible and may become necessary to give the constant componentof premagnetization during the break step a magnitude different fromthat effective during make performance. This can simply be obtained bymeans of an additional direct-current premagnetization of the reactorcore. When employing transductors for producing the trapszoidal current,the direct current for the excitation of the transductor reactors maysimply be passed through an additional winding on the core of thecommutating reactor.

To permit, in addition to maximum-voltage adjustment of the currentconverter (zero delay angle), also a voltage regulation by delay-anglecontrol (delayed-commutation method), the eifectiveness of thevoltageu'espom sive component or also of the constant component, or ofboth components of the make premagnetization is delayed to such a degreethat the make step can only commence immediately after the contactclosing moment or so shortly ahead of this moment that after the closingmoment a still sufficient portion. of the make step remains availablefor the temporary suppression of the current rise.

The desired delay in the initiation of one or both premagnetizingcomponents may be effected by a controllable valve, for instance, a gridcontrolled gaseous or vaporous discharge device. Especially suitable forthis purpose is a cesium vapor discharge device tube because of itssmall arc drop voltage. The delay in voltage inception may also bemagnetically produced, for instance, by a so called valve reactorconsisting of a series connection of a direct-current excited saturablereactor with an electric valve preferably of the two-electrode type(transductor or magnetic amplifier). In both cases the initiation of theexcitation current must be placed into the proper time relation to theclosing moment of the converter contact. In contact converters withmotor-driven contacts, the driven contact member or a separatepreclosing contact, for instance, may serve to supply the grid of thecontrollable valve with a positive control pulse for igniting the valve.In current converters with electromagnetically actuated contacts, theignition pulse for the controllable valve in the premagnetizing circuitmay be made dependent upon the pulse for controlling the closingoperation of the converter contact.

Another way of controlling the delay in inception of the premagnetizingcurrent is available in converters whose contacts are controlledelectromagnetically by a separate control circuit of the type describedand claimed in the copending application of E. Rolf and M. Belamin,Serial No. 278,386, filed March 25, 1952, and assigned to the assigneeof the present invention. Such a separate control circuit, beinggalvanically, inductively or capacitively coupled with the mainconverter circuit and containing a current control device such as agrid-controlled tube or a variably excited transductor, operates toimpart a controllable delay to the contact closing operation; andaccording to a feature of the present invention may simultaneously servefor controlling the initiation of the premagnetizing current for thecommutating reactor.

For reliable converter performance, it is important to have thecommutating reactor in condition for break performance during thepredominant portion 'of each alternating-voltage cycle period. Incontrast, the reactor need be prepared for make performance only duringa small portion of the period, this portion being determined by theparticular circuit scheme of the converter plant and by the range ofvoltage regulation to be effected by delayed-commutation control. Forthis reason the break premagnetization according to the invention isgiven predominance. That is, the duration of the make premagnetizationwithin the alternating-voltage period is kept shorter than the durationof the break premagnetization.

The foregoing and other objects, advantages and features of my inventionwill be apparentfrom, or will be referred to in, the followingdescription of the embodiments of the invention exemplified by thedrawings, in which:

Fig. 1, explained above, is a coordinate diagram concerning the magneticbehavior of a commutating reactor in a contact converter;

Fig. 2 is a schematic single-phase circuit diagram of a contactconverter;

Fig. 3 shows five interrelated coordinate diagrams (a) to (e)explanatory of the operation of a converter according to Fig. 2;

Fig. 4 is a modification of the single-phase contact converter shown inFig. 2;

Fig. 5 shows a schematic circuit diagram of a modified three-phaseconverter; and Fig. 6 presents a set of coordinate diagrams forexplaining the operation of a converter according to Fig. 5;

Fig. 7 is a partial single-phase circuit of another converter, and Fig.8 is a pertaining explanatory voltage diagram;

Fig. 9 shows another partial converter phase circuit, and Fig. 10 is anexplanatory voltage diagram relating to Fig. 9;

Fig. 11 shows essentially a known basic bridge circuit of a three-phasefull-wave converter and is presented according to the invention;

Figs. 13 and 14 are respective circuit diagrams of two more elaboratelydesigned three-phase converters, and

Fig. 14a shows separately a circuit detail of the converter according toFig. 14;

Fig. 15 illustrates eight explanatory coordinate diagrams (a) to (h) ofcurrent and voltage conditions typical for converters according to Figs.14 to 18; and

Figs. 16, 16a, 16b, 17 and 18 are schematic circuit diagrams ofrespective further modifications of contact converters.

In all illustrations the same reference characters are used for denotingrespectively similar elements or magnitudes.

The single-phase converter circuit shown in Fig. 2 includes, as itsalternating-voltage source, the secondary winding 1 of a powertransformer energized from an alternating-current supply line.Series-connected in the circuit are the mainwinding 3 of a saturablecommutating reactor 2, a synchronous contact device 4 having a movablecontact element engageable with two stationary contact elements, and adirect-current load 5. The contact device 4 will hereinafter be brieflyreferred to as the contact. The contact is actuated to open and close insynchronism with the wave of the alternating voltage and to this end isdriven by a suitable means schematically indicated by a broken line 10.This drive means,

as illustrated, comprises a control magnet with a holding coil 44 and acontrol coil 44'. The holding coil 44 is series connected in the maincircuit of contact 4 and reactor winding 3 to be energized by thecontact current. It need not be inserted at the particular place shownin Fig. 2 but may instead be connected at another place of thecontact-controlled phase circuit, for instance, between the circuitpoints 0 and d.

It should be understood that converters according to the invention mayhave contact controlling or actuating means other than that shown inFig. 2. For instance, the actuating means 10 may also consist of a camshaft driven from a synchronous motor energized from transformer winding1 through a phase shift transformer.

As explained, the commutating reactor, due to the reversal of themagnetization of its saturable core, produces during each cycle period astep in the current wave during which the contact 4 may open withoutsparking. For improving the shape of the current step, the reactor islinked with an auxiliary shaping circuit 17 comprising an auxiliary coil16 on the reactor core in connection with a combination of capacitanceand resistance, with or without inductance. In its simplest case, forinstance, the circuit of coil 16 may essentially be composed of acapacitor and a calibrating resistor, as illustrated.

As explained in the foregoing, my invention requires applying to thecommutating reactor a prernagnetization comprising a constant componentand a variable component of which the latter is dependent upon thevoltage across the series connection of the main reactor winding and theconverter contact. According to Fig. 2, the constant component of thebreak premagnetization is applied to the reactor 2 by means of anauxiliary winding 6 which is disposed on the saturable reactor core and.energized from any suitable source 7 of direct voltage,- such as abattery, through a smoothing and stabilizing reactor 9 and through anadjusting or calibrating resistor 8. This constant-component circuit as'a whole is denoted by 27. The magnetomotive force produced by thiscircuit in the reactor 2 is determined by the constant ampere turns ofauxiliary winding 6. The voltage-responsive component of the breakpremagnetization is applied by the reactor by an auxiliary winding 3a onthe reactor core. Winding 3a lies in a premagnetizing cirudes anadjusting o c a n res s or n sefis'" with EN aIve IZL T he electrornotive force preduced by i cuit 32 in reactor 2 is variable in accordancewith the pessi ism-11's of auxiliary winding 3a.

The two component electromotive forces of the break pro magnetizationoppose the'rnagnetization of the reactor core hy the load currentflowing through reactor windh1g3. when these two premag'netizingcomponents are alone effective, the commutating reactor is always inconditioir for sparkle's s break operation. The preparation jfor themake .operationfhowever, represents a temporary'exception from thisgeneral condition and takes place only when, during the normal course ofthe voltage wave, the conditions are just suitable for contact closirig,that is, when there is a voltage capable of driving the current in thedesired direction through the converter contact A. ror this purpose theconstant component of the make premagnetization'in'the converteraccording to 12 is 'p tied means of an auxiliary circuit 52 ih cqlj illlfitlon with automatically operating devices which phase position ofthis premagnetizing component relative to the energizing alternatingvoltage in depehdence upon the closing moment. This automatic phaseshift device consists essentially of the series connection of acapacitor 53 with the working winding 55 of a ,transductor 54 to which acharging circuit and a discharging circuit are connected. The chargingcircuit cQn 'lprises an half-wave rectifier 59, a current-limitingresistor 63 and the secondary winding 62 of an auxiliary transformerwhose primary 61 is connected to the energizing alternating voltage ofthe power transformer winding 1. The discharge circuit is formed by anauxiliary circuit 52 which includes an auxiliary winding 33 on the coreof the commutating reactor 2, and a grid-controlled discharge device Thecapacitor 53 is charged during each negative half wave of thealternating voltage and discharges itself, after release by tube 25,through the auxiliary winding 33, thus issuing to that winding a currentpulse whose amplitude is kept constant by the transductor 54. For thispurpose the core of transductor 54, (representing a saturable reactor orsaturable transformer) is magnetically excited by its control winding 56up to far above its saturation knee. The control winding 56 is connectedthrough a stabilizing reactor 57 and an adjusting or calibratingresistor 58 to a suitable source of direct voltage 69, hereschematically represented by a battery, the polarity of connection beingsuch that the magnetizing effect of control winding 56 is opposed tothat of the discharge current flowing in the main winding 55 oftransductor 54. Under these conditions, the discharge pulse has such amagnitude that it just balances the direct-current premagnetization.Therefore, by adjusting the direct-current premagnetization with the aidof resistor 58 the magnitude of the current pulse and hence themagnitude of the constant component of the make premagnetization canreadily be adjusted to the proper value.

The voltage-responsive component of the make premagnetization issupplied by an auxiliary circuit 42, which, for' -instance, may excite aseparate auxiliary winding 13 on the core of the commuta tin'g reactor.The circuit 42 includes an adjusting resistor 14 and may alsobe'equipped with a half waye rectifier 15', such as a barrier layerrectifier. The circuit 42 may further be connected to the discharge tube25 at point g instead of being connected at point 1 as illustrated.

In the case of motor-actuated converter contacts, the grid of tube 25may be controlled by the drive itself, particularly "by the drivepertaining to the converter contact in the same phase circuit. To thisend, the grid circuit of tube 25 may be connected to a preclosingcontact or to the movable QOllVGI'tQI contact itself in such a mannerthat the auxiliary circuit 42 becomes closed a small fraction of amillisecond ahead of the closing moment of the converter contact '4proper, or at the latest 'tion' toward the flux axis.

together with the converter contact. This will be more fully'zdescribedbelow in conjunction with Fig. 14. i

Instead of being connected to the grid-controlled tube 25, the'auxiliarycircuit 42 may also be connected to another discharge device or, asillustrated, to a transductor 10b. Within its unsaturated range, themagnetic characteristic of this transductor has an appreciable inclina-The transductor 10b is preexcited by a stabilized direct-current circuit65 in such amanner that it operates in an unsaturated initial conditionwhen its main winding is not traversed by current. A regulating resistor66 permits varying the pro-excitation to adjust the initial condition oftransductor 10b to any desired point between the two saturation knees ofits magnetic characteristic. Series connected with the main winding oftransductor ltlb is the primary winding of a small saturable transformer67 whose core is magnetically excited by direct current in the knownmanner and whose secondary winding provides the, control voltage for thegrid of tube 25 in coaction with a series connected source 68 of directvoltage. The control circuit of winding 44 may also be connected withthe transductor 10b, this circuit including a valve 49, a resistor 41and an auxiliary source 47 of constant bias voltage. The bias voltage isso dimensioned that, with contact 4 closed, a sufficient flow'of controlcurrent is maintained through winding 44' to'hold contact 4 in closedposition.

For explaining the operation of the converter, reference will be made inthe following to the diagrams shown in Fig. 3. In Fig. 3(a) the curve Urepresents the voltage of the power transformer (winding 1 in Fig. 2).The horizontal line V denotes the constant component of the breakpremagnetization (supplied by the constant ampere turns of auxiliarywinding 6 in Fig. 2) eficctive during the entire operating period of theconverter. The voltage U, ascending from zero, has at first the effectof placing the magnet core of the transductor 10b from its unsaturatedinitial condition into the saturated state. The necessary voltage-timeintegral is represented by the area F and is indicative of the degree ofvoltage control (commutation delay angle) of the converter plant. Thecontrol degree therefore can be regulated by means of the resistor 66,After the transducer 10b is saturated, it releases a how of currentthrough its main winding. The increase of this current imparts throughthe switching transformer 67 an ignition pulse to the grid tube 25 sothat the previously charged capacitor 53 discharges a constant currentthrough the auxiliary winding 33 of the commutating reactor 2, thussupplying this reactor with the constant component V of makepremagnetization. Simultaneously, the flow of current initiated throughtransductor 10!; supplies the voltage-responsive component V N of themake premagnetization according to Fig. 3(b) and passes through theauxiliary winding 13 of reactor 2. The released flow of transductorcurrent also provides the control current i for the control winding 44'of the contact control magnet according to Fig. 3(a). The rise ofcontrol current i is delayed by the inductivity of the winding 44. Assoon as the current 1 exceeds the critical pick-up value of the magnetsystem, the contact 4 closes at the moment E. Shortly thereafter themake step ceases so that the load current I increases to its full valueaccording to Fig. 3(d). This moment denotes the end of the delayinterval or control angle on. Thereafter, the control current i declinesto a value which, at a corresponding adjustment of resistor 41, liesabove the drop-off value of the control magnet. The load current I isshown in Fig. 3(d) for a load circuit of purely ohmic character. Throughwinding 44 this current exerts an additional holding force whichvanishes when the'load current wave reaches its zero value and entersinto the break step. At first, however, the converter contact is stillkept closed due to the control current i flowing in winding 44'.

The time curve of the voltage-responsive component V of the breakpremagnetization issliown' in Fig. 3(0). This component, as mentionedabove, is proportional to the commutating voltage, the proportionalityfactor being half as large during the break step as at a later stage.The voltage, which at the beginning of the break step occurs across thecommutating reactor winding 3, is opposed to the driving alternatingvoltage and has practically the same magnitude as the latter. In thepresent case, therefore, the absolute values of the voltage acrosswinding 3 increase from the zero value and act in opposition to thedriving voltage from the battery or constant voltage source 47. Thisreduces the current i in the control circuit-from the inception of thebreak step until the current 1' reaches zero. As soon as the decliningcurrent i subsides below the critical holding value of the controlmagnet, the converter contact opens at the moment A. Consequently, thecontact can open only during a break step interval.

The fact that in the converter according to Fig. 2 the release of thepremagnetizing circuit 52 is made dependent upon the initiation of thecontrol current for the con-' tact closing operation, results in thefurther advantage that the constant component of the makepremagnetization and hence the occurrence of any make premagnetization,is

completely prevented it due to any cause of trouble the control currentfor the contact closing operation should fail to appear.

Instead of providing the core of the commutating reactor 2 with anauxiliary winding 3a for supplying the voltage-dependent component ofbreak premagnetization as described in the foregoing with reference toFig. 2, a portion of the reactor main winding 3 may also be used forthis purpose. It is then preferable to connect one half of the mainwinding into the'auxiliary premagnetizing circuit because this reducesthe losses in that circuit to a minimum.

to the auxiliary circuit 52 for supplying the constant components V;; ofthe make premagnetization (auxiliary reactor winding 33). In this case,the lattercomponent, as well as the voltage-responsive component, may bedirectly supplied from the contact-closing voltage as soon as thetransductor 10b reaches its saturated condition. Also in this case, themagnitude of the constantcomponent is determined by the preexcitation ofthe transductor 54. It is also possible, alternatively, to pass bothcomponents of the make premagnetization current through the sameauxiliary winding 13 of the commutating reactor thereby eliminatingwinding 33.

A converter according'to Fig. 4 operates fundamentally in-the samemanner as the above-described converter according to Fig. 2 That is, theconverter of Fig. 4 also affords a fully automatic adaptation of themake premagnetization to the particular moments'of contact operation.The adjustment or regulation of the delivered voltage by delayedcommutation is again effected with the aid of the resistor 66. In viewof the fact that the auxiliary circuit for providing the constantcomponent of the make premagnetization requires a relatively largeexpenditure in circuit elements, this component may be eliminated forlesser requirements, for instance, when an only small power rating or asmall range of voltage regulation is demanded. In the latter case, thevoltage-responsive component of the make premagnetization is to beincreased accordingly.

Another possibility according to the invention for prodiicing theconstant component of the make premagnetizaly coincide with the zeroaxis.

vIt) I tion is available in three-phase converter circuits Fig. 5exemplifies this additional possibility."' The respective circuitspertaining-to *thethre'fbha's R, S, T of the converter shown in Fig.Shave thesame design and performance, except that the respective conjtact operations and the correlated clfects in the three phases occur ina cyclical sequence and with the proper phase spacing. For that reason,the reference numerals mentioned below are applied in Fig. 5 only tomeals mentsthat either pertain to the phase R or are common to all threephases. It may be mentioned at thispoint that the embodiments shown assingle-phase circuits (Figs. 2, 4, 7, 9, 16 to 18,) are, of course, alsoapplicable to multiphase converters in an analogous manner.

In the three-phase Y-connected converter ofFig. 5.the

direct-current circuit comprisesa load 5 in series'with' a smoothingreactor 5'. be controllable by a circuit breaker 37, a shunt-connectedbase load 5" may also be provided in the customary man ner. Theauxiliary circuit 52 which energizes the aux iliary winding '33 of thecommutating reactor 2 is connected to the secondary Winding 1 of thepower transformer through a half-wave rectifier 45, a stabilizingreactor 29 and an adjusting resistor 28. An auxiliary secondary winding1a of the power transformer is series connected in the circuit 52 foreach phase so that the auxiliary voltage of each winding In, due tozig-zag interconnection of the windings 1a with the main windings R, S

and T, is somewhat leading with respect to the voltage of its respectivemain Winding 1. The auxiliary winding 33 of reactor 2 in circuit 52 hasthe polarity of connection required for its ampere turns (electromotiveforce) to act in opposition to that of the auxiilary winding ,6 forproducing the constant component V;; of the make premagnetization. Themagnitude of the premagnetizing cur rent in winding 33 is so adjustedthat during the interval available-for the make performance theinstantaneous cur rentvalues are approximately twice as high as thepremagnetizing current in winding 6, assuming both windings to have thesamenumbcr of turns. The voltage-responsive component of the makepremagnetization is supplied by the auxiliary circuit 42 with thegrid-controlled discharge tube 25. The control circuit for the grid oftube 25 (not illustrated in Fig. 5), could for example be the sameas'that illustrated'in Fig. 14(a) and described below in con nection withthe operation of the circuit shown in Fig. 14.

The grid circuit is operative 'to control tube 25 to initiate thevoltage responsive component V of the make magnetization shortly beforethe closing moment of the main contact. l I

The operation of the converter according to Fig. 5 may be more fullyexplained,with reference to the diagrams of Fig. 6, under the assumptionthat the rectified output current of the converter is completelysmoothed by the reactor 5. The upper portion of Fig. 6 shows the timecurve" U U U of the main voltages of the power-transformer secondaries 1in the respective converter phases R, S, T. The upper portion-of Fig. 6'further shows the corresponding phase currents (main currents I I Iflowing through the respective converter contacts at a control angle ofapproximately a'=4()., the limit being at about a =90 for this example;The current steps practicah The make moments E and the break moments Aof the converter contacts lie within the current steps.

The next lower portion of Fig. 6 represents the break premagnetization.The constant component V (auxiliary circuit 27) is indicated by ahorizontal line below the zero axis and is emphasized by verticalcross-hatching This component is continuously present as long as theconverteris in operation. The voltage-responsive corn ponent V(auxiliary circuit 32) is illustrated for the If, as shown, the loadcircuit is toreactor at the zero passage of current I at the beginningof the break step, one-half of the commutation voltage U becomeseflective in the auxiliary circuit 32 if this circuit, as shown, isconnected to the midpoint of the commutating reactorwinding 3. This isso because at this moment the full commutation voltage is effectiveacross the entire winding 3. Now the break step interval takes itscourse. the converter contact 4 of phase R opens.

At the moment A of the break step However, a magnetizing currentcontinues to fiow through the auxiliary current path 32. This continuedcurrent prolongs the premagnetization of the commutating reactor inphase R in coaction with the constant component V of-thebreakpremagnetization and without change in the magnetornotive-forcedirection until the commutating reactor is saturated and hence the breakstep terminated. From this moment on, the commutation voltage U isshifted from across the reactor winding 3 to across the opened contactdevice 4 thus making the full volt age effective in the auxiliarycircuit 32. Due to the predominantly ohmic impedance of auxiliarycircuit 32, the premagnetization V is at any moment proportional tow thedriving voltage that produces this premagnetizati on (factorConsequently, during the break step V -=c-U /2, and immediately afterthe break stepv V R',= saparent from the drawing. During the currenttransfer (commutation) between the phases S and T, that is duringthe-interval of time in which the contacts of these two phases are bothclosed and the respective commutating reactors are both saturated, theauxiliary circuit 32 is subjected to 1.5 times the amount of the voltageof phase R; and after the phase S enters into the break step, theinterphase voltage U becomes effective in the auxiliary circuit 32pertaining to the phase R. Essential for this time course of the breakpremagnetization V 'is the fact that it acts in the same direction asthe constant component V i. e. in opposition to the magnetomotive-forcecaused by the main current flowing through the converter contacts.Consequently, although the starting moment of the break step may occurat any desired moment, the converter affords the security that this steppractically coincides with the current zero line except during theinterval of time within which the contact is to close.

On the other hand, the make premagnetization is ef fective only duringsuch an interval of time in which generally the conditions are suitablefor the closing of the converter contact, the voltage then having thedirection needed for driving the current in the desired directionthrough the contact. 7 With a normal voltage characteristic, theseconditions are satisfied for the contact in phase R from the moment whenthe ascending phase voltage U becomes larger than the descending phasevoltage U At this moment, first the constant component V of the makemagnetization becomes effective in the auxiliary circuit 52, if at firstthe leading angle of the here efiective driving voltage is disregarded.The bottom portion of Fig. 6 represents this make premagnetization ofthe commutating reactor in phase R. The magnitude V of this makepremagnetization has about The further course is immediately aptwice thevalue of the constant component V of the 12 I nations entered into thebottom diagram of Fig. 6 refer to absolute values. The curve shape ofthe constant component V;; of the make premagnetization, in the presentcase, results from the fact that the auxiliary circuits 52 of the threephases are combined with the pertaining valves 45 to a three-phaserectifier arrangement in Y-connection. The duration of the constantcomponent of the make premagnetization within the alternating voltagecycle-in this example is limited to the interval of time within whichthe closing moment may be shifted for voltage control by delayedcommutation. The curve shown by a heavy full line whose area isvertically crosshatched in the bottom diagram of Fig. 6, has a leadingphase relation to the corresponding curve shown by a broken line, thisbeing due to the above-mentioned phase combination involving theadditional transformer windings 1a.

The voltage-responsive component V of the make premagnetization wouldimmediately commence at the end of the voltage-responsive breakpremagnetization V apparent from the middle diagram of Fig. 6) and wouldalso immediately follow the voltage U (or now U if the valve 12 were notprovided. This valve, however, makes certain that after the occurrenceof a voltage acting in the contact'closing sense, there will at first bea complete cessation of voltage-responsive premagnetization. Thispremagnetization is released only by the subsequent ignition of the tube25 at, or shortly prior to, the closing moment of the converter contact4 and is then eflective in the contact closing sense, namely in the samedircction'as the main current. This causes the voltage U to appearacross the commutating reactor winding 3 so that the contactclosingtakes place free of voltage and the flow of main current is at firstprevented until the commutating reactor saturates in the contactclosingsense. Only. then can the current commutation commence with a steep riseof the current I Consequently, the above-mentioned electrical or othercoupling, such as the mechanical coupling of the control for tube 25with the device that closes the converter contact, has the result thatthe converter system prepares itself for the closing operation only whenthe conditions are such that the operation may actually be carried outwithout danger to the contacts.

However, the last-described modification does not afford such a fargoingreduction in the duration of the make premagnetization as is possible ifthe constant component of this .premagnetization is put into action witha phase position automatically adapting itself to the contactclosingmoment as is the casein the embodiments previously described withreference to Figs. 2 and 4.

In all above-described embodiments of the invention theconstantcomponent of the break premagnetization is brought about by a directcurrent on which an opposing premagnetization for the make performanceis superimposed within a limited interval of each cycle period. Instead,and as introductorily mentioned, a premagnetization of alternatingdirection, preferably of trapezoidal wave shape, may also be applied,for instance, withthe aidof an auxiliary rectifier circuit or a seriestransductor comprising two opposingly pre-excited saturable reactors.

Exac ting requirements as regards reliability and safety of operationcan be satisfied by properly selecting not only the direction but alsothe magnitude of such a preexcitation, preferably independently for themake per formance on the one hand and the break performance on the otherhand. This prevents any adjustment of the pre-excitation for the makeperformance from being aecompanied by an undesired or possibly evendangerous misadjustment for the break performance, and vice versa.

The desired independence can beobtained, when using a seriestransductor, by mutually adapting a direct-current pre-excitation of thetransductor and the opposingly acting transductor pulse. Accordingly,the communtating rcactors in the embodiments described below areequipped with a premagnetizing circuit which contains the main windingof a series transductor (hereinafter briefly called transductorcircuit), as well as with a direct-current premagnetizing circuit whichincludes the direct-current coils of the same transductor, the lattercircuit being so rated or tuned that it balances the opposingly actingpremagnetizing pulse from the transductor circuit.

Modifications of converters according to the invention embodying thejust-mentioned features are illustrated in Figs. 7 and 9 in conjunctionwith the respective explanatory diagrams of Figs. 8 and 10. Figs. 7 and9 show only a portion of a circuit pertaining to one phase of theconverter circuit. Complete converter circuits involving such circuitportions are described in a later place.

Fig. 7 shows a commutating reactor 2 with a saturable magnet core whosemain winding 3 is series connected with the converter contact 4. Atransductor 70 is providedfor premagnetizing the commutating reactor.The transducer 70 in this example is composed of two similar saturablereactors whose main windings 71 and 72 are series connected with thepremagnetizing windings 6 of the commutating reactor. The transductorcircuit is connected to a source of alternating voltage of suitablemagnitude and phase position. This voltage may be taken from theenergizing voltage of the converter, for instance, with the aid of anauxiliary transformer in a multiphase connection and preferably with aphase combination or zig-zag connection similar to the correspondingcircuit ,devices described with reference to Fig. 5. The directcurrentexcitation windings 71' and 72 of the transductor are connected inseries opposition. Series connected with windings 71 and 72 are anotherpremagnetizing winding 6" of the commutating reactor, a stabilizingreactor 9 and an adjusting resistor 8. This direct-current circuit isenergized from a suitable voltage source 7 exemplified by a battery. Theillustrated premagnetizing device may serve for producing aconstantcomponent of the break premagnetization. The arrowhead enteredupon the main circuit is supposed to indicate the direction of thecurrent to pass through the converter contact 4 when the latter isclosed.

During the operation of the device, a premagnetizing current i, flowsthrough the transductor circuit, the time curve of this current beingrepresented by the idealized rectangular wave shown in Fig. 8. Thecurrent i passing through winding 6, produces alternately positive andnegative premagnetizing pulses V and V-;- of the same absolutemagnitude, and both having the same duration equal to a half-wave periodof the alternating voltage. Superimposed on these pulses by means of thewindings 6" is a direct-current premagnetization V which balances thepositive pulses V This requires that the ratio of the winding turns w /wof windings 6 and 6' be in accordance with the transformer ratio w/w' ofthe working windings (main windings) to the direct-current windings ofthe transductor. V is a unipolar, namely negative pulse of the durationof a half wave. This pulse is apparent from Fig. 8 if the line denotedby 0 is taken as the zero line. By changing the resistance adjustment ofresistor 8 the pulse magnitudes V and V' as well as the amount V arechanged to the same extent. Consequently, the ultimate result of such achange in adjustment is always a unipolar, negative pulse except thatits absolute value is different. In the period of time between twopulses, any other magnetizing components that may possibly be presentare not affected by a change in resistance adjustment of resistor 8.

Another premagnetizing device is shown in Fig. 9. This device may servefor providing a constant component of the make premagnetization in formof a unipolar positive pulse of a duration shorter than a half wave.This modification involves subject matter also disclosed in thecopending application of M. Belarnin, Serial No. 311,395, filedSeptember 25, 1952. The negatively pre-excited The resultingpremagnetization I saturable reactor 72/72 of thetransductor in thismodification has a smaller transformation ratio w /w and hence is morestrongly excited than the positively excited saturable reactor 71/71whose transformation ratio w /w is made larger, for instance, by using asmaller number of winding turns in the pertaining directcurrent winding71'. In this case the winding 6" and its premagnetizing circuit may atfirst be disregarded.

Fig. 10 shows the time curve of the premagnetizing current i in thetranductor circuit. This current supplies positive pulses of themagnitude V and negative pulses of a smaller magnitude V Added to thesepulses is the direct-current premagnetization V in the positivedirection and of the same magnitude as V- Hence the negative pulses Vare balanced so that unipolar, positive pulses V counted from the line0, will remain. This requires that the winding turn ratio w /w be equalto the Winding turn ratio w /w' of the windings 71 and 71 that determinethe negative pulse. Such a make premagnetization with unipolar positivepulses may be used, for instance, together with a break premagnetizationaccording to Figs. 7 and 8, provided the phase relation is properlyadapted so that the phase position for each device may be variedindependently of the other. In cases where such a mutual independence isnot required, the direct-current circuits of the two premagnetizingdevices may be combined in a common direct-current circuit whichincludes all direct-current windings in series connection. Similarly, inmultiphase converters, all direct-current circuits in premagnetizingdevices of the same kind pertaining to the different phases may becombined with one another without disturbing the independence fromcircuits for other kinds of premagnetizing components.

A simplification, in comparison with the above-described mutuallyindependent premagnetizing circuits may often be found permissible fromthe following considerations. It is impuortant that a change inadjustment of V does not affect the magnitude of V because even atemporary increase in magnitude of.V may lead to arcback at theconverter contact. A temporary variation of V however, does not endangerthe reliability of operation but may at most have a slight efiect uponthe occurrence of transfer of contact material. Accordingly, the deviceshown in Fig. 9 is supplemented by the winding 6' with a pertainingseparate direct-current circuit comprising a direct-current source 7such as a battery, a stabilizing reactor 9 and an adjusting resistor 8'.According to Fig. 10 this circuit provides a continuously effectivenegative direct-current premagnetization in the amount of the requiredbreak premagnetization V Superimposed on this continuous magnetizationare unipolar pulses as described. The magnitude V',; of the pulses isso-adjusted that after deduction of the amount V the requiredpremagnetization V will exist. Generally the value V may be somewhatlower than for the break performance as is apparent from Fig. 10. Whenin the device according to Fig. 9 the magnitude of the makepremagnetization V is changed by means of resistor 8, the change doesnot affect the break premagnetization. When changing the breakpremagnetization V,, by adjusting the resistor 8, the value of V willalso vary, namely as may be deduced from Fig. 10, by the same amount andin the opposite sense. However, this does not appreciably impair theoperation and may safely be corrected by a subsequent readjustment of VAnother component circuit device, incorporated in the more comprehensiveembodiments still to be described, or applicable as a modification inconverters otherwise similar to those described above, relates to themeans for supplying the commutating reactor with the variablevoltage-responsive component of premagnetization. The

circuit meansso far described for producing the voltageresponsivepremagnetization may be used in any singlephase and' multiphaserectifier connections in which each converter contact has its owncommutating reactor (halfwave rectifiers). However, there are alsorectifier systems in which one and the same commutating reactor servesto produce the break step of two converter contacts operating inpush-pull, namely during positive and negative half waves respectivelyof the alternating current to be rectified (full-wave rectifiers). Thisis the case, for instance, in three-phase bridge connections with sixconverter contacts and only three commutating reactors, this so calledthree-reactor scheme being at present predominant in contact converterswith motor-driven contacts. The basic scheme is represented in Fig. 11.y

In Fig. 11 the secondary windings I 1 I of the power supply transformerare connected through the respective reactor windings S 3 3-;- onrespective saturable cores 2 2 2 with pairs of converter contacts 4;;and 4' 4 and 4' 4 and 4 The contacts are interconnected by a bridgecircuit and the two contacts of each pair operate in push-pull. A load 5is connected in the output branch of the bridge circuit to be energizedby full-wave rectified current. I

The above-described circuit connection for producing thevoltage-responsive premagnetization component cannot readily be appliedin such a bridge type rectifier circuit. If one attempted to connect thevoltage-responsive premagnetizing circuit at one end to the transformerside of a reactor main winding and at the other end to the load side ofthe pertaining converter contact, as is entered in Fig. 11 by a dottedline, then the two premagnetizing circuit branches 11/12 and 11712leading to the positive and to the negative output buses respectively ofthe converter would operate as a voltage divider with respect to therectified output voltage, and the premagnetizing winding 3a of thecommutating reactor would then be connected to the voltage midpoint ofthis voltage divider. The resulting voltage and current conditions wouldfundamentally depart from those desired and necessary for the properoperation of the rectifier. In fact, due to the preloading of the valves12 and 12' by the direct current flowing from the positive to thenegative bus through the two branch circuits 11, 12 and 11, 12', thevalves 12 and 12' would not be capable of directing the premagnetizingcurrent in the one-half wave to contact 4 and in the other half wave tocontact 4' According to another feature of my invention, however, theconnection for the voltage-responsive premagnetizing circuit may be somodified that it satisfies the requirement of properly premagnetizingthe commutating reactor prior to the contact opening moment withoutincurring the detrimental voltage-dividing efiect explained inconnection with Fig. 11.

To achieve this improvement, I connect in a multiphase contact rectifierthe voltage-responsive premagnetizing circuit of one phase to the phasenext following in the commutation sequence. In other words, thevoltageresponsive premagnetizing circuit forms a cross-phase" circuitinstead of the along-phase circuit of the embodiment so far described.In connection therewith, and as already mentioned, a portion of the mainreactor winding may be used as a premagnetizing winding. In a bridgeconnection with only one commutating reactor for two converter contactsoperating in push-pull, the provision of the cross-phase connection ofthe voltage responsive premagnetizing circuit obviates the valve '12required in the corresponding along-phase connection described for thepreceding embodiments. However, the valve 12 must be retained inconverters with one contact per commutating reactor even if thevoltage-responsive premagnetizing circuit is cross-phase connected.

A three-reactor connection with cross-phase connected circuits for thevoltage-responsive component of the break premagnetization is shown inFig. 12. The illustrated positions of the converter contacts 4 to 4correspond to the condition existing shortly prior to he opening ofconact '4 Consequently, contact 4 has ing 3 pertaining to the incipientphase.

already closed and carries direct current from phase S to the positivepole of the load 5, while the likewise closed contact v4 passes thedirect current from the negative pole of load 5 back to phase T of thepower transformer. At this stage the core 2 of the cormnutating reactorin phase S ha been saturated from the inception moment .of the loadcurrent in phase S so that no voltage is efiec- .tive across the reactorwinding 3 Consequently, the contact 6 the contact 4 the positive pole ofload 5, and the winding 3 of the reactor in phase S are all on the samepotential, namely on the potential of the terminal of transformerwinding I It is therefore irrevelant, for the time characteristic of thepremagnetiza' .tion during the break step interval of a decaying phase,at which particular circuit point between the transformer terminal ofthe incipient phase and the alternatingcurrent side of the contacts inthe decaying phase, the premagnetizing circuit branch is connected.According to Fig. 12, for instance, this point of connection lies at thetransformer side of the commutating reactor wind- Since at the instantunder consideration the contacts 4' and 4 are open, no connectionbetween the premagnetizing circuit and the negative direct-current poleexists during the break step for the commutating reactor in the decayingwave to the positive direct-current pole and in the other half wave tothe negative pole is automatically effected by means of thecorresponding contacts of the incipient phase. This is also the reasonwhy, as mentioned, particular valves are not required for this controlpurpose. The entire voltage-responsive premagnetizing circuit thereforeconsists only of the premagnetizing winding 3a,

' (or, instead, a portion a of the reactor main winding) and the lowinductance ohmic resistor 11 The constant component of thepremagnetizing flux is produced with the aid of the auxiliary winding 6.Be-

cause of the double utilization of the commutating reactor for producingbreak steps in both half waves, this constant component ofpremagnetization cannot be preduced by direct current but requires anexcitation of alternating polarity and preferably of trapezoidal waveshape 'as described previously.

When using a single break-step reactor for two, pushpull contactsaccording to Fig. 12, this reactor cannot also be used for producing themake steps. It is then necessary to provide either a separate makereactor or at least a separate make core which has the reactor mainwinding in common with the pertaining break core. In both cases thecommutation reactors have separate make cores. The difference in thefunctioning of the make cores from that of the break cores is determinedby their difierent premagnetization.

separate make cores is preferably also composed of a Thepremagnetization of the constant component and a voltage-responsivelyvariable component, the phase position and magnitude of these componentsbeing, of course, different from those of the break cores.

The embodiment illustrated in Fig. 13 exemplifies the just-mentionedfeatures in a three-phase full-wave (bridge connected) rectifier. Therectifier is connected to a source of alternating voltage here againconsisting of the 17 during the respective make and break steps toreduce the instantaneous current practically to zero. From the reactormain winding 3 of each phase the anode lead branches to two convertercontacts 4 and 4' of which the former is connected to the positive busand the latter to the negative bus of the direct-current load circuits.The two contacts are either motor driven or are electromagneticallycontrolled in dependence upon the instantaneous values of the current orvoltage. The actuating means are schematically indicated by a dot anddash line 10. The direct-current circuit includes a load 5 and may alsobe equipped with a stabilizing series reactor and such auxiliary devicesas shown, for instance, in the load circuit illustrated in Fig. 5.

The make core 2 receives a constant premagnetizing component through anauxiliary winding 33, and also a voltage-dependent component through anauxiliary winding 13, both auxiliary windings being disposed on core 2but not inductively linked with the core 2. During make performance,both components have the same magnetizing direction as the main currentwinding 3. The break core 2 receives corresponding premagnetizingcomponents through the auxiliary windings 6 and 3a which are inductivelylinked only with the latter core. Both components act during the breakinterval in a magnetizing direction opposed to that of the main current.The constant premagnetizing component is furnished by an auxiliarycurrent of alternating direction and trapezoidal wave shape which, inthis embodiment, is supplied from a three-phase symmetrical seriestransductor 70. The phase position of the trapezoidal current may beadjusted by means of a suitable phase combination, for instance, and asshown, by a transformer 70a energized from the secondary windings 1 ofthe power transformer. The auxiliary transformer 70a may be substitutedby a rotary phase-shift transformer to permit a supplemental adjustmentin phase position. One and the same trapezoidal current may be used forthe two reactor cores 2' and 2. Accordingly, the auxiliary reactorwinding 6 and 33 are shown to be series connected. Under the conditionthat the make and break cores have the same average diameter and consistof the same magnetizable material and have the same magnetizingbehavior, the turn number of winding 33 on the make core 2 is preferablysomewhat smaller than the turn number of winding 6 on core 2.

The excitation windings of transductor 70 are all series connected andare energized from a source 7 of directcurrent, exemplified by arectifier arrangement, through an adjusting resistor 8 and a stabilizingreactor 9.

The auxiliary circuit of winding 13 supplying the make core with thevoltage-responsive premagnetizing component, is branched into twocircuit portions with respective resistors 14, 14 and respective valvesand 25 of mutually inverse connection polarities. This auxiliarycircuit, as a Whole, lies parallel to the series connection of thereactor main winding 3 and the respective converter contacts 4, 4. Thevalves are controlled with the aid of such grid circuit means asrepeatedly mentioned in the foregoing or as described hereinafter inconjunction with Figs. 14 and 140, so that the auxiliary currents in thewinding 13 occur each time either simultaneously with the contactclosing moment or shortly prior to that moment, depending upon whetherthe entire make step or only a rest portion thereof is supposed to occurafter the make moment. This is determined by the voltage control angle(commutation-delay angle) of the rectifier plant and the correspondingmagnitude of the commutation voltage which obtains during .the makeperformance and reverses the magnetization of the reactor core 2' duringthe make step interval.

The voltage-dependent component of the break premagnctization is appliedto the auxiliary winding 3a through a resistor 11 by the voltage betweenthe desaying and incipient phases of the alternating current source 1that participate in any particular commutation. These parts form across-phase circuit in zig-zag connection as explained in the foregoing.The operation of the rectifier according to Fig. 13 is also inaccordance with the foregoing explanations and hence requires no furtherdiscussion.

The following explanationsrelate to further improvetive) until thereactor core reaches saturation. When' the premagnetizing currentceases, the magnetizing condition of the commutating reactor returns tothe negative point of remanence. Without special expedients therefore,the entire step would have to be traversed in the opposite directionafter the next reclosing of the In general, this is undesired because ofthe resulting reduction .in

contact, before the load current could rise.

voltage and power factor. To avoid this deficiency, it is necessary tohave the resaturation of the commutating reactor in the make direction(positive direction) at least partially occur at a moment sulficientlyahead of the next contact closing moment.

If, further, separatecores or even separate complete commutatingreactors are provided for producing the respective make and break stepsin combination with the aforedescribed premagnetizing, controlling andother auxiliary circuits, then care must be taken, for instance with theaid of additional premagnetizing circuits, that at the closing of thecontacts the pertaining break core of the commutating reactor issaturated, and at the opening of the contacts the pertaining make coreis saturated, so that neither core can interfere with the properfunctioning of the other.

To this end the premagnetizing devices may be modi-. tied, for instance,in such a manner that the resulting magnetomotive force of the reactorcore for producing the break step has, during a limited interval of timewithin each period, a magnetizing direction suitableforback-magnetization in the make sense and that then the magnitude ofthis resulting magnetomotive force has a value above the staticremagnetizing value and drops at least as far as to this static valueand preferably even down to zero before the occurrence of the contactclosing moment. If such a device is equipped with a separate make core,the premagnetization of the make core may also be modified in a similarmanner. 1

The static remagnetization value H, according to Fig. 1 has one definitevalue for a reactor core of highquality magnetic material whosecharacteristic in the unsaturated range runs parallel to the flux axis.For a magnet core of lower-quality material a correspondingly definitevalue of the magnetomotive force may also be fixed, for instance, bymeans of a stretching or shaping circuit as described in the foregoing(27 in Figs. 2, 4). However, even without shaping circuits, theabove-described requirement can reliably be met also withcommutating-reactor cores of lesser magnetic quality if the resultingmagnetomotive force during the mentioned limited time interval is largerthan the highest magnetomotive-force value of static remagnetization asindicated by the ordinate value of the saturation knee in the ascendingbranch of the magnetization charac teristic, and if further theresulting magnetomotive-force declines to the lowest staticremagnetization value that is practically to zero, before the occurrenceof the contact closing moment.

The back-magnetization of the commutating reactor has the eflfectofsupplying it prior to the make operation with a voltage integral of agiven magnitude which appears as an area in the voltage time diagram,this voltage l id integral; having, the, function of remagnetizing; thecom-.

mutating reactor whollyor partially. in the direction of the positivesaturation; at a time ahead; of, the closing moment.

"the; required amount of the back-magnetization may differ dependingupon the particular circuit scheme of the w lverter. Ifin theabovementioned group of converter connections. with only one contact,per commutating, reactor a: separate make core is provided, then themaincore (break core), as a rule, is always completely remagnetized.upto positive saturation. Similar conditions apply analogously to themake core. On the other hand if one; and the same core serves forproducing,thema ke; step as well as the break step, and if a voltageregulation, by, mechanical control (delayed commutation byphase-shifting the contact closing moment) and. especially with a largeangle of commutation delay, is desired, then much of theback-magnetization should, be anti-eipated prior to. the closing momentso that, only, a relatively. short residual: portion of the make step.can, occur after the closing moment. As a result, the step. interval;which. otherwise would have an undesirably long durationv because of thelow values of commutating voltage obtaining in this case, becomeseffective, only during an interval of the desired short duration. Inbothabove-mentioned, cases of mechanical voltage control the supply. of avoltage area (voltagetime integral), of. invariable magnitude isgenerally involved. In. contrast thereto, the magnetic voltage control(involving a, magnetic control of the commutating reactors) whichoperates with a make step of variable length, requires that, themagnitude of the back-mag netizing voltage area (voltage-time integral)be regulatableto a largeextent,

It is advantageous to give the backrmagnetization the character of apulse of limited duration because, for instance with respect to thebreak core, this magnetization must commence only after the cessation ofthe break step and should terminate before the closing moment, so thatthis pulse at that moment can no longer impose avoltage on thecommutating reactor thus permittingthe contact to close free of voltage.For that reason, only a portion of thecycle period of notmuch more than60 electrical is generally available for applying the back-magnetizingpulse- In principlqany kindofjpulseis. suitable as abackmag netizingpulse-provided it is capable-of supplying the commutating reactor withinthe available cycle portion with a voltage area of'the requiredmagnitude, the, particular wave shape of the pulse being not essential.

During the back-magnetization, the current in the pulse circuitcorresponds to the step current of the commutating reactor plus anamount required for. compensating any additionally present opposinglydirected premagnetiza-tion.

One possibility accordingto the invention of producing suchback-magnetizing pulses consists again in the application of anodecurrents from rectifier arrangements. Because of the limitation of theback-magnetizing interval, 'a six-phaseY-connection of the auxiliaryrectifiers is-preferably applicable for this purpose,

Another way-of producing back-magnetizing pulses isoficred-by'theapplication ofithe auxiliary devices according to Figs. 7and-9; permittingthe production of pulses ofa. duration shorter than 60electrical.

Theconverter shown in Fig.- 14 exemplifies the backmagnetizationfeaturesexplained inthe foregoing. The illustrated 'converterhas athree-phaseY-connected circuit'energizedfrom the secondarywinding 1 of a powertransformer whoseprima-ry 1 has its terminals R, S, T, connected to. athree-phase supply line. Connected to the secondary side are the threephases=R, S, T of the converter; circuit. Thel three phase circuits havethe same: design. For simplicity, the pertaining reference charactersare'indicatedin only one of-the phases. Each phasecomprisesthemainwinding of acommutating 2%) reactor in; series; with, a; synchronous.converter contact. driven, for instance, by an; actuating: device 1 0;from a synchronous motor-Mia toperiodically. close and open in therhythm of the alternating phase voltage. Thev load. circuit isrepresented by aload' device, 5: in. serieswith: a, smoothing reactor.5.

Each commutating reactor has a break core; 2, and a separate make core2". The main reactor winding 3 is inductively coupled with both cores,and; each core is. additionally equipped; with several auxiliary.windings. While the magnet cores themselves are omitted fromthe drawing,a dot-and-dash enclosure shown. for each, rc-- actor core of the phase Sindicates which of the auxiliary windings are inductively linked withthebreak core- 2: and which other auxiliary windings are linked. withthemake core 2. Besides, the manner of illustration is so chosen that thecurrent iiowdirection in the windings. also indicates the direction ofthe, magpetomotiveforce: produced in the pertaining cores. 'llhe;downward direction on the. drawing istaken as;positiv,e,f

Four auxiliary windings 6,, 6,", 3.6, and 22 are linked only with. thebreak core, 2.. Of these, the. windings 6 and 6'. together supply an.invariable component V of the break premagnetizatiomand. alsotheback-magnetiz-ation V of. the. break core. The winding 6 is. seriesconnectedv with the active, winding of an. asymmetrical seriestransdu-CIQr-ZO whichisattached. to the, energizing phase leads. R, S, Tthrough, a phase-shift device here exemplified by an. adjustablephase-shift transformer. The winding 6v is further combined with. thecorresponding windings of the two other phases to-form a Y-connection.The, direct-current windings of the transductors, as apparent from, thedrawing, are series connected in pairs, the two windingsuof each pairhaving a mutually opposedpoling, These direct-currentwindings, are,energized from a direct-current source 7 shown, for example. as abarrier-layer rectifier arrangement, energized, from phase leads R, S, Tthrough an'auxiliary transformer. The direct-current circuit isstabilized by a reactor 19. The two transductor portions of eachphasehave different ratios of winding turns (transformer ratios), thisbeing indicated in the drawing by different lengths ofthe symbols forthe direct-current windings. In transductor 20 the upper portion ispositively pie-excited by the direct current to a larger degree than thenegatively pro-excited lower portion. Consequently, this transductorsupplies alternating pulses whose positive wave is lower and longer thanthe, negative wave. The winding 6 is.so connected that the polaritiesare reversed. Hence, winding 6'produccs a short and high positive pulsewhich etiects the remagnetization, and a long pulse which negativelypremagnetizes the break core during thepred'ominant portion of eachcycle period and-in the sense of readiness for break performance. By asuitable choice of the transformer ratio of the two transductorportions, the positive back-magnetizing pulse can be, made to extendover onlyv about 60 electrical, Forthis purpose, the transformer ratiosmust be related about 60:30Q=1:5. The phase portion (i. e. in theillustrated example the control of the phase-shift transformerpertaining to the device, 20) is so adjusted that the back-magnetizingpulse is terminated shortly prior to the contact closing-moment (seeFig. 15(f) explained below). The magnitude ofthe pulse is so chosen thatthe pulse suffices for completely backrnagnetizing the break core frompositive to negative saturation. The adjustment is etfected by changingthe direct current with the aidgof an adjusting resistor 13. Thisadjustment aiso determines the magnitude of the negative pulse in thepremagnetizing. winding 6.. The latter pulse magnitude, ingenerai, isnot suflicientfor the fixed component V of. the break premagnetizationwhich issupposed to correspond tothe value ,H in Fig; 1. However, themissingramount of; negative magnetomotive. force is supplied by theauxiliary winding 6" which: may also be connected to the direct current:source 7 through.

an adjusting resistor 8 and a stabilizing reactor9. The

positive back-magnetizing pulse must be adjusted by a higher amountequal to the magnitude of this continuously effective negativemagnetomotive force.

The make core 2' receives the fixed component V of the makemagnetization through the windings 33 and 33'. Winding 33 is traversedby alternating pulses from the arrangement 40 of asymmetrical seriestransductors. The positive wake' portion of these pulses is higher andshorter than the negative portion because the upper half of thetransductors, which determines the positive pulsewave portion, is morestrongly pre-exicted than the lower transductor portion. In winding 33the negative wave of this pulse is just balanced by an additionalpositive cur-rent through the pertaining winding 33'. To secure thisbalance, the winding 33 is series connected with the direct-currentwindings of the transductor device 40, and the turn number of winding33' is related to that of winding 33 in the same ratio as thedirect-current to alternating-current transformation ratio of the lowertransductor half-section to the corresponding transformation ratio ofthe upper transductor half-section. In this manner the resultant effectof the winding group 33, 33' is a unipolar, positive pulse, acting inthe.sense of the make performance and having a duration, with respect tothe remaining length of a cycle period, determined by theabove-mentioned ratio of the turn numbers, i. e. the transformationratio of the two transductor halfsections. The magnitude of the pulse isadjustable by the resistor 28 in the direct-current circuit. Thisdirect-current circuit is also equipped with a stabilizing reactor 29.The duration and the phase position of the make magnetization are to bechosen so that the pulse extends at least over the range of phasepositions that the make step may occupy during the normal operation ofthe converter (see Fig. 15 (e) explained below). This is obtained by acorresponding dimensioning of the turn numbers of the transductorwindings or/ and a corresponding selection of the phase position of thesupplied auxiliary voltage, for instance as illustrated, by means of thephase-shift transformer pertaining to the device 40.

For back-magetizing the make core 2 a unipolar pulse in the negativedirection is applied with the aid of windings 23 and 23'. Winding 23receives alternating pulses from the transductor device 50. Winding 23receives direct current, as it is series connected with thedirect-current excitation windings of the transductor device 50, and hasits turn number dimensioned in the above-described manner relative tothe winding 23. The phase position of the negative back-magnetizingpulse for make core 2' may be chosen that this pulse immediately followsthe latest operationally possible time position of the break step. Thecorresponding direct-current circuit contains an adjusting resistor 38and a stabilizing reactor 39.

The voltage-dependent component V of the make magnetization is suppliedby an auxiliary circuit with a valve 25 and an adjusting resistor 14.This circuit is attached to the midpoint of the main winding 3 of thecommutating reactor and includes a decoupling winding 22 disposed on thebreak core 2 and acting on this core in opposition to the magnetizationeffected by the upper half of winding 3. The valve 25 is shown as agrid-controlled tube. The grid circuit 34' of tube 25 may have anysuitable design, for instance, the one separately illustrated in Fig.14a. The grid circuit, as shown, is connected to a preclosing contact 4"so that the voltage responsive component V of the make magnetizationcommences each time shortly before the closing moment of the maincontact to act with a positive direction in the upper half of thecommutating reactor winding 3. In the grid circuit design exemplified byFig. 14a, a source 46 of grid bias voltage serves to apply a negativecut-ofi potential. Another source 48 of a higher voltage changes theresultant grid potential to a positive magnitude for.

. the make performance.

22 firing the tube 25 as soon as the contact 4a closes. The grid circuitfurther includes a current limiting resistor 43, a capacitor 68 and adamping resistor 64.

For producing the voltage responsive component V N or" the breakmagnetization, an auxiliary circuit with an adjusting resistor 11 and apreferably non-controllable (two-electrode) valve 12 is provided. Thisauxiliary circuit is also connected with the midpoint of the commonreactor main winding 3 and extends through a decoupling winding 22linked only with the make core 2. Due to the chosen fiow direction ofthe valve, this auxiliary circuit comes into action during the breakstep. Further details of the functioning of the auxiliary circuits forthe voltage-responsive premagnetizating components V and V are presentedbelow in conjunctionwith Fig. 15.

Each of the two cores 2 and 2' may further be equipped with one of theabove-mentioned shaping circuits. Such a shaping circuit is illustrated,by way of example, for the break core and is denoted by 17. This shapingcircuit is connected to the auxiliary winding 16 of the commutatingreactor.

Figs. 16 to 18 show three respective modificationsby partialillustrations. In these modifications each commutating reactor has onlyone core for producing the current steps for the break performance aswell as for These modifications may not only be used in a three-phaseY-connection as illustrated, but are also applicable without change in athree-phase bridge (full-wave) connection with six reactors; and theyare also adaptable for other converterconnections with one contact percommutating reactor. The converter contacts may be driven by synchronousmotors or may be controlled electromagnetically as described in theforegoing. The modifications of Figs. 16 to 18 relate to converters foran exclusively magnetical voltage regulation. Circuit adaptations forvoltage regulation by magnetical and mechanical control will be referredto in a later place. The corresponding time characteristic of thevoltages and currents in the illustrated converter circuits areschematically represented in Fig. 15

In Figs. 16 to 18, the means for supplying the fixedly predeterminedcomponents of the make and break premagnetization for each phase aresymbolically indicated as a winding 60. The winding symbol 60 is meantto denote any of the previously described devices for producing aresultant magnetomotive force of alternating direction with a preferablyrectangular curve shape. As apparent from the foregoing, the productionof such a magnetomotive force may require several auxiliary windings onthe commutating reactor core. In this sense, therefore, the symbol 6!)may denote a single winding as well as a group of several coactingwindings.

The illustrated embodiments are further equipped with anotherpremagnetizing winding or winding group 30 on the commutating reactor ineach phase for controlling the back-magnetization as described in theforegoing.

Before dealing with further details of the just-mentioned modifications,it appears preferable to first discuss the phase conditions of thepremagnetizing components in converter connections of the type shown in'Figs. 14 and 16 to 18 will be further explained wit-h reference to Fig.15. Fig. 15 (a) represents the time curves of the Y-voltages e e e inthe respective phases R, S, T of the power transformer. Fig. 15(1))shows schematically the typical Wave shape of the load currents i i iwhich pass through the commutating reactor main winding and theseries-connected converter contact in the respective phases. In theillustrated example, the voltage control angle at, extending from theintersection of any two successive phase-voltage curves to the moment atwhich the two respective load currents commence to overlap, amounts toabout 40 e In the case of a magnetic voltage, regulation, the entireangle interval is occupied by the make step. In Fig. 15 (a) the voltagearea D -is correlated to this make step in the phase ..R. Thedirect-current voltage. at. time-refill follows. the'curve e Immediatelyfollowing the make ,step .is .:the:overlapping interval ofthe loadcurrents. During this interval the .direct voltage is in accordance withthe -middle curve between the voltages e and 2 The voltage area Dremains maintained by the inductivity of thesaturated commutatingreactor of phase R, while the voltage area T is maintained by thetransformer and line inductivity of the same phase. Thereafter,thecurrent 1' enters into the break step. From nowon, the direct voltagecorresponds to the curve e During the break step interval the voltagearea D is efiective atthe commutating reactor of phase T; During thesubsequent current commutation from phase R to phase S, the samephenomena reoccur in a corresponding cyclical substitution. The voltageareas D of the commutating reactor and T of the transformer phase-R nowlie below the middle curve of the direct-voltage. 'Whilethis is of noconcern for the constant component of premagnetization, it issignificant for the followingconsideration of the time course of thevoltage-dependent break premagnetization.

Fig. 15 (c) shows the curve of the magnetomotive force for theapplication of a symmetrical transductor. The interval of the invariablecomponent V of break premagnetization must begin not earlier than at 90and not later than at 120. After a duration of 180, this interval passesat 270 to 300 into the interval of the invariable component V of makepremagnetization. Since the make interval begins ahead of the time pointa=0, the magnitude of the fixedly predetermined magnetomotive force mustnot exceed the value required for the static remagnetization ('H inFig. 1) to prevent causing an undesired, premature back-magnetization.The back-magnetization should exclusively be controlled by theback-magnetizing pulse V -which takes place in the time interval fromthe end of the break pre magnetization V to the time point i=0, asisrepresented in Fig. 15(f).

Fig. 15(d) shows one of the possible courses of the fixed premagnetizingmagnetomotive force for the case that in one of the above-mentionedconverter connections an asymmetrical series transductor isused,-possibly with an additional compensation of the negative curveportions. The interval of the make premagnetization V in this casecommences at the point a=0 or shortly ahead of this point, andterminates at about 90", then to be followed by the interval of thebreak premagnetization V,, which occupies the entirerest of the cycleperiod. The backemagnetizing pulse has the phase position shown in'Fig.15(1) but, in this case, must be considerably higher, as corresponds tothe case of Fig. 15 (0) because the pulse must now balance the breakpremagnetization and must additionally furnish the magnitudecorresponding to the invariable make prem-agnetization. If, instead of atransductor circuit, a rectifier connection with 120 anode-currentduration is employed for producing the premagnetizing magnetomotiveforce, then the interval of the make premagnetization has the courseentered in Fig. 15(d) by a broken line.

Fig. 15 (e) represents the time curve of the invariable premagnetizingcomponents V and V;, as they would occur when unipolar pulses ofmutually opposing polarity are employed for the make and breakperformance as described'in the foregoing. In this case, theback-magnetizing pulse V need no longer balance the.composivclymagnetic-voltageregulation, ;the:vo1tage-responsive premagnetizingcircuits. include only non-controlled (twoelectrode) a valves 15 :orxnovalves ;at all,:;asthe inception of the currentforthevoltage-responsivemake premagnetization practically.coincides withthe contact-closing moment occurringsat-the time point m=0 or only afewdegrees after that point. ,In circuit arrangements, however which. arealso intendedvto operate with a voltage regulation.by"mechanicaldelay-angle control, the circuits for thevoltage responsive .makepremagnetization must be equippedwith. controlled valves in order tovprevent.the premature expirationof the make step that may otherwiseoccur .under the influence of both make premagnetizing components V andV N In the embodiment of Fig. 16, two. separate adjusting resistors 14and 11 are providedfor the'voltage-responsive make. premagnetizingcircuit and for .the voltage responsive break premagnetizing circuitrespectively. This is necessary1if,:for. optimum. adaptation, thecomponents V -rand-.V "have;differentmagnitudes. The time curve ofthercurrents in the two voltage-responsive premagnetizingcircuitsisrepresented in Fig. 15(g). The.make premagnetizing current,proportional to the component V commences at the timepoint a=0 inWhich-the phase voltages e and 2 are equal. Thence, :this premagnetizingcurrentincreases during the make stepproportionally to=a sine curve (notseparately illustrated) corresponding to thevoltage difference ebetween-e and 0 The pertaining voltage area is indicated in Fig. 15a atD It should be noted that onlyhalf of the voltage e occurs at theprernagnetizing circuit here under observation if this circuit, asillustrated, is attached to the-midpoint of the commutating reactor mainwinding 3. During theoverlapping interval of the load currents, thepremagnetizingcurrentdeclines to a curve corresponding to the=inductivevoltage at the saturated commutating reactonof phase R, the pertainingvoltage area being indicated in Fig. 15(a) atD When the currentcommutation .is completed, the'premagnetizing current ceases becausethen no voltage remains effective across the commutating reactor winding3 of phase R.

The break premagnetizing current corresponding to component V Ncommences atthe moment when due to the current commutation of loadcurrents i and i an inductive voltage of the opposite polarity occurs atthe commutating reactorof phase R, the corresponding voltage areabeingdenoted'byDfln Fig.- 15 (a). After-termination of the currentcommutation, this premagnetizing current rises to-a -value proportionalto 'the'commutatin reactor voltage-obtaining during thebreak step.During this step, the commutatingreactor' has a voltagedrop e (notseparately illustrated) between e and 2 corresponding to the voltagearea D in Fig. 15(a). Accordingly, half of the voltage 2 is effective inthe premagnetizing circuit here concerned. After termination of thebreak step, the-premagnetizing current follows the course of voltage ehowever with a doubled magnitude because now the ohmic resistance 'ofthe premagnetizing circuit is no longer impressed by half the cut-oifvoltage e but, upon-saturation of'the commutating 'reactor,-is subjectedto the full value -of this voltage. The premagnetizing current-then runsaccording-to the curve of'the voltagee upto'the end'of thecycle'rperiod. During the last interval WhiChiS avail-able for aback-magnetization, for instance according to'Fig. 15 (f), anadditional-voltage is superimposed upon the sinusoidal basic curve "ofthe cut ofi voltage e The superimposed voltage'corresponds'tohalf thevoltageoccurring at the commutating-reactor during theback=magnetization, assuming that the back-mag etizing pulse hastheefiect of anticipatingpart ofthe break interval prior tothe moment-e= 0.The just-mentioned voltage superposition is indicated in Fig. 15 (g) 'bya broken line.

The embodiment-of Fig. 17 differs'trom that ofFig. 16 in'that only oneresistor 11 is employed for the two volt-

